1. Field of the Invention
This invention relates to apparatus for operating a gas discharge lamp, such as a fluorescent light, a mercury vapor lamp, a sodium lamp, or a metal halide lamp.
2. Related Applications
This application discloses subject matter which is related to U.S. patent application Ser. No. 865,209, Filed Dec. 28, 1977, now U.S. Pat. No. 4,168,453, issued Sept. 18, 1979; Ser. No. 940,435, Filed Sept. 7, 1978; and Ser. No. 968,372, filed Dec. 11, 1978; all by Francis Henry Gerhard and Gerald Allen Felper; all for "VARIABLE INTENSITY CONTROL APPARATUS FOR A GAS DISCHARGE LAMP".
3. Description of the Prior Art
Control circuits for gas discharge lamps are known which obviate the need for the usual heavy and expensive series ballast devices, corresponding to the inductor in this device. In such circuits, switching elements are provided to periodically reverse the direction of current through the lamp to reduce the deterioration or errosion of electrodes, and to ensure a high enough frequency of switching to reduce the requirement for the size of the ballast. Such circuits generally require two switching elements for each direction of the current.
Attempts have been made to fabricate the same type of circuit using only a single switching element to cause current reversal in the lamp. For example, the U.S. Pat. to D. B. Wijsboom, No. 3,906,302, is directed to such an arrangement and incorporates an inductor in parallel with the lamp, which lamp is in series with a switching device. Such a switching device is generally operated at relatively high frequencies, such as 20 kHz.
One problem has been that the illumination intensity of the lamp for a given amount of power consumed is maximized only if the switching device operates to provide a symmetrical voltage wave form to the lamp. Typically, the magnitude of the voltage supplied to the circuit determines the shape of the voltage wave form supplied to the lamp. As a result, in general, there is a specific voltage which must be supplied to the circuit in order to provide a symmetrical voltage wave form to the lamp. The applicant has emperically found that the power efficiency of the lamp is maximized only when a symmetrical voltage wave form is supplied to the lamp, and that, for a high intensity mercury vapor lamp connected to a control circuit having a single switching element, a voltage supplied to the circuit of approximately 130 volts DC when warmed up, or 20 volts DC when cold, results in a symmetrical wave form. The problem of maximizing the efficiency of the lamp by providing a fixed supply voltage which ensures a symmetrical voltage wave form in the lamp is compounded because, if the control circuit is designed to provide the requisite 130-volt DC value for a symmetrical voltage wave form in the lamp after warm up, then the time require to warm up the lamp after initial turn-on would be extended to become excessively long, and it is even possible that the lamp, after initial turn-on, would never reach its normal operative mode.
Another problem is that, even though the control circuit may be designed to apply the requisite voltage to ensure a symmetrical voltage wave form in the lamp, the requisite voltage may change during the life of the lamp due to change in lamp characteristics, and is different from lamp to lamp due to manufacturing tolerance variations. Furthermore, changes in lamp characteristics may result in a change in load impedance presented to the power supply, which may cause a change in the voltage output of the power supply, further complicating the task of attempting to supply the requisite voltage required to ensure a symmetrical voltage wave form in the lamp. Furthermore, power loss in the power supply itself occurs if the power supply input impedance is reactive. Finally, even if the power supply is designed to provide the requisite voltage to the lamp for corresponding to a symmetrical voltage wave form in the lamp, variations in the voltage in the power line supplying power to the power supply may cause the power supply to vary its voltage output from the desired requisite voltage.